Monochromatic x-ray systems and methods

ABSTRACT

According to some aspects, a carrier configured for use with a broadband x-ray source comprising an electron source and a primary target arranged to receive electrons from the electron source to produce broadband x-ray radiation in response to electrons impinging on the primary target is provided. The carrier comprising a housing configured to be removeably coupled to the broadband x-ray source and configured to accommodate a secondary target capable of producing monochromatic x-ray radiation in response to incident broadband x-ray radiation, the housing comprising a transmissive portion configured to allow broadband x-ray radiation to be transmitted to the secondary target when present, and a blocking portion configured to absorb broadband x-ray radiation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 120 and is acontinuation of U.S. application Ser. No. 15/984,174, filed May 18,2018, entitled MONOCHROMATIC X-RAY IMAGING SYSTEMS AND METHODS, whichclaims priority under 35 U.S.C. § 119 to U.S. Provisional ApplicationSer. No. 62/508,996 filed May 19, 2017, and titled MONOCHROMATIC X-RAYSYSTEMS AND APPARATUS, each application of which is herein incorporatedby reference in its entirety.

BACKGROUND

Traditional diagnostic radiography uses x-ray generators that emitX-rays over a broad energy band. A large fraction of this band containsx-rays which are not useful for medical imaging because their energy iseither too high to interact in the tissue being examined or too low toreach the X-ray detector or film used to record them. The x-rays withtoo low an energy to reach the detector are especially problematicbecause they unnecessarily expose normal tissue and raise the radiationdose received by the patient. It has long been realized that the use ofmonochromatic x-rays, if available at the appropriate energy, wouldprovide optimal diagnostic images while minimizing the radiation dose.To date, no such monochromatic X-ray source has been available forroutine clinical diagnostic use.

Monochromatic radiation has been used in specialized settings. However,conventional systems for generating monochromatic radiation have beenunsuitable for clinical or routine commercial use due to theirprohibitive size, cost and/or complexity. For example, monochromaticX-rays can be copiously produced in synchrotron sources utilizing aninefficient Bragg crystal as a filter or using a solid, flat targetx-ray fluorescer but these are very large and not practical for routineuse in hospitals and clinics.

Monochromatic x-rays may be generated by providing in series a target(also referred to as the anode) that produces broad spectrum radiationin response to an incident electron beam, followed by a fluorescingtarget that produces monochromatic x-rays in response to incident broadspectrum radiation. The term “broad spectrum radiation” is used hereinto describe Bremsstrahlung radiation with or without characteristicemission lines of the anode material. Briefly, the principles ofproducing monochromatic x-rays via x-ray fluorescence are as follows.

Thick Target Bremsstrahlung

In an x-ray tube electrons are liberated from a heated filament calledthe cathode and accelerated by a high voltage (e.g., ˜50 kV) toward ametal target called the anode as illustrated schematically in FIG. 1.The high energy electrons interact with the atoms in the anode. Often anelectron with energy E₁ comes close to a nucleus in the target and itstrajectory is altered by the electromagnetic interaction. In thisdeflection process, it decelerates toward the nucleus. As it slows to anenergy E₂, it emits an X-ray photon with energy E₂-E₁. This radiation iscalled Bremsstrahlung radiation (braking radiation) and the kinematicsare shown in FIG. 2.

The energy of the emitted photon can take any value up to the maximumenergy of the incident electron, E_(max). As the electron is notdestroyed it can undergo multiple interactions until it loses all of itsenergy or combines with an atom in the anode. Initial interactions willvary from minor to major energy changes depending on the actual angleand proximity to the nucleus. As a result, Bremsstrahlung radiation willhave a generally continuous spectrum, as shown in FIG. 3. Theprobability of Bremsstrahlung production is proportional to Z², where Zis the atomic number of the target material, and the efficiency ofproduction is proportional to Z and the x-ray tube voltage. Note thatlow energy Bremsstrahlung X-rays are absorbed by the thick target anodeas they try to escape from deep inside causing the intensity curve tobend over at the lowest energies, as discussed in further detail below.

Characteristic Line Emission

While most of the electrons slow down and have their trajectorieschanged, some will collide with electrons that are bound by an energy,BE, in their respective orbitals or shells that surround the nucleus inthe target atom. As shown in FIG. 4, these shells are denoted by K, L,M, N, etc. In the collision between the incoming electron and the boundelectron, the bound electron will be ejected from the atom if the energyof the incoming electron is greater than BE of the orbiting electron.For example, the impacting electron with energy E>BE_(K), shown in FIG.4, will eject the K-shell electron leaving a vacancy in the K shell. Theresulting excited and ionized atom will de-excite as an electron in anouter orbit will fill the vacancy. During the de-excitation, an X-ray isemitted with an energy equal to the difference between the initial andfinal energy levels of the electron involved with the de-excitation.Since the energy levels of the orbital shells are unique to each elementon the Periodic Chart, the energy of the X-ray identifies the element.The energy will be monoenergetic and the spectrum appears monochromaticrather than a broad continuous band. Here, monochromatic means that thewidth in energy of the emission line is equal to the natural line widthassociated with the atomic transition involved. For copper Kα x-rays,the natural line width is about 4 eV. For Zr Kα, Mo Kα and Pt Kα, theline widths are approximately, 5.7 eV, 6.8 eV and 60 eV, respectively.The complete spectrum from an X-ray tube with a molybdenum target as theanode is shown in FIG. 5. The characteristic emission lines unique tothe atomic energy levels of molybdenum are shown superimposed on thethick target Bremsstrahlung.

X-Ray Absorption and X-Ray Fluorescence

When an x-ray from any type of x-ray source strikes a sample, the x-raycan either be absorbed by an atom or scattered through the material. Theprocess in which an x-ray is absorbed by an atom by transferring all ofits energy to an innermost electron is called the photoelectric effect,as illustrated in FIG. 6A. This occurs when the incident x-ray has moreenergy than the binding energy of the orbital electron it encounters ina collision. In the interaction the photon ceases to exist imparting allof its energy to the orbital electron. Most of the x-ray energy isrequired to overcome the binding energy of the orbital electron and theremainder is imparted to the electron upon its ejection leaving avacancy in the shell. The ejected free electron is called aphotoelectron. A photoelectric interaction is most likely to occur whenthe energy of the incident photon exceeds but is relatively close to thebinding energy of the electron it strikes.

As an example, a photoelectric interaction is more likely to occur for aK-shell electron with a binding energy of 23.2 keV when the incidentphoton is 25 keV than if it were 50 keV. This is because thephotoelectric effect is inversely proportional to approximately thethird power of the X-ray energy. This fall-off is interrupted by a sharprise when the x-ray energy is equal to the binding energy of an electronshell (K, L, M, etc.) in the absorber. The lowest energy at which avacancy can be created in the particular shell and is referred to as theedge. FIG. 7 shows the absorption of tin (Sn) as a function of x-rayenergy. The absorption is defined on the ordinate axis by its massattenuation coefficient. The absorption edges corresponding to thebinding energies of the L orbitals and the K orbitals are shown by thediscontinuous jumps at approximately 43.4 keV and 29 keV, respectively.Every element on the Periodic Chart has a similar curve describing itsabsorption as a function of x-ray energy.

The vacancies in the inner shell of the atom present an unstablecondition for the atom. As the atom returns to its stable condition,electrons from the outer shells are transferred to the inner shells andin the process emit a characteristic x-ray whose energy is thedifference between the two binding energies of the corresponding shellsas described above in the section on Characteristic Line Emission. Thisphoton-induced process of x-ray emission is called X-ray Fluorescence,or XRF. FIG. 6B shows schematically X-ray fluorescence from the K shelland a typical x-ray fluorescence spectrum from a sample of aluminum isshown in FIG. 8. The spectrum is measured with a solid state, photoncounting detector whose energy resolution dominates the natural linewidth of the L-K transition. It is important to note that thesemonoenergetic emission lines do not sit on top of a background of broadband continuous radiation; rather, the spectrum is Bremsstrahlung free.

SUMMARY

According to some embodiments, a carrier configured for use with abroadband x-ray source comprising an electron source and a primarytarget arranged to receive electrons from the electron source to producebroadband x-ray radiation in response to electrons impinging on theprimary target is provided. The carrier comprising a housing configuredto be removeably coupled to the broadband x-ray source and configured toaccommodate a secondary target capable of producing monochromatic x-rayradiation in response to incident broadband x-ray radiation, the housingcomprising a transmissive portion configured to allow broadband x-rayradiation to be transmitted to the secondary target when present, and ablocking portion configured to absorb broadband x-ray radiation.

Some embodiments include a carrier configured for use with a broadbandx-ray source comprising an electron source and a primary target arrangedto receive electrons from the electron source to produce broadband x-rayradiation in response to electrons impinging on the primary target, thecarrier comprising a housing configured to accommodate a secondarytarget that produces monochromatic x-ray radiation in response toimpinging broadband x-ray radiation, the housing further configured tobe removably coupled to the broadband x-ray source so that, when thehousing is coupled to the broadband x-ray source and is accommodatingthe secondary target, the secondary target is positioned so that atleast some broadband x-ray radiation from the primary target impinges onthe secondary target to produce monochromatic x-ray radiation, thehousing comprising a first portion comprising a first materialsubstantially transparent to the broadband x-ray radiation, and a secondportion comprising a second material substantially opaque to broadbandx-ray radiation.

Some embodiments include a monochromatic x-ray device comprising anelectron source configured to emit electrons, a primary targetconfigured to produce broadband x-ray radiation in response to incidentelectrons from the electron source, a secondary target configured togenerate monochromatic x-ray radiation via fluorescence in response toincident broadband x-ray radiation, and a housing for the secondarytarget comprising an aperture through which monochromatic x-rayradiation from the secondary target is emitted, the housing configuredto position the secondary target so that at least some of the broadbandx-ray radiation emitted by the primary target is incident on thesecondary target so that, when the monochromatic x-ray device isoperated, monochromatic x-ray radiation is emitted via the aperturehaving a monochromaticity of greater than or equal to 0.7 across a fieldof view of at least approximately 15 degrees. According to someembodiments, monochromatic x-ray radiation emitted via the aperture hasa monochromaticity of greater than or equal to 0.8 across a field ofview of at least approximately 15 degrees. According to someembodiments, monochromatic x-ray radiation emitted via the aperture hasa monochromaticity of greater than or equal to 0.9 across a field ofview of at least approximately 15 degrees. According to someembodiments, monochromatic x-ray radiation emitted via the aperture hasa monochromaticity of greater than or equal to 0.95 across a field ofview of at least approximately 15 degrees.

Some embodiments include a monochromatic x-ray device comprising anelectron source configured to emit electrons, a primary targetconfigured to produce broadband x-ray radiation in response to incidentelectrons from the electron source, and a secondary target configured togenerate monochromatic x-ray radiation via fluorescence in response toincident broadband x-ray radiation, wherein the device is operated usinga voltage potential between the electron source and the primary targetthat is greater than twice the energy of an absorption edge of thesecondary target. According to some embodiments, the device is operatedusing a voltage potential between the electron source and the primarytarget that is greater than three times the energy of an absorption edgeof the secondary target. According to some embodiments, the device isoperated using a voltage potential between the electron source and theprimary target that is greater than four times the energy of anabsorption edge of the secondary target. According to some embodiments,the device is operated using a voltage potential between the electronsource and the primary target that is greater than five times the energyof an absorption edge of the secondary target.

Some embodiments include a monochromatic x-ray device comprising anelectron source comprising a toroidal cathode, the electron sourceconfigured to emit electrons, a primary target configured to producebroadband x-ray radiation in response to incident electrons from theelectron source, at least one guide arranged concentrically to thetoroidal cathode to guide electrons toward the primary target, and asecondary target configured to generate monochromatic x-ray radiationvia fluorescence in response to incident broadband x-ray radiation.According to some embodiments, the at least one guide comprises at leastone first inner guide arranged concentrically within the toroidalcathode. According to some embodiments, the at least one guide comprisesat least one first outer guide arranged concentrically outside thetoroidal cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and embodiments of the disclosed technology will bedescribed with reference to the following figures. It should beappreciated that the figures are not necessarily drawn to scale.

FIG. 1 illustrates a schematic of a broadband x-ray source;

FIG. 2. illustrates the scenario in which an electron (much lighter thanthe nucleus) comes very close to the nucleus and the electromagneticinteraction causes a deviation of the trajectory where the electronloses energy and an X-ray photon is emitted and describes Bremsstralungin its simplest form;

FIG. 3 illustrates the Bremsstrahlung spectrum produced by a typicalX-ray tube, wherein the lower energy x-rays trying to escape the targetare absorbed causing the characteristic roll over of the spectrum at lowenergies;

FIG. 4 illustrates the physical phenomenon that generates characteristicline emissions;

FIG. 5 illustrates the combined spectrum from an X-ray tube with amolybdenum anode showing the thick target Bremsstrahlung and thecharacteristic molybdenum line emission;

FIG. 6A illustrates the photoelectric effect;

FIG. 6B illustrates the principle of X-Ray fluorescence from the Kshell;

FIG. 7 illustrates the absorption coefficient as a function of x-rayenergy for tin, wherein the discontinuous jumps or edges show how theabsorption is enhanced just above the binding energies of the electronsin tin;

FIG. 8 illustrates an X-Ray fluorescence spectrum made by irradiating atarget of aluminum (Al) with copper x-rays which were generated by anx-ray tube with an anode of copper;

FIG. 9 illustrates an x-ray apparatus for generating monochromaticx-rays;

FIGS. 10A and 10B illustrate on-axis and off-axis x-ray spectra of x-rayradiation emitted from a conventional monochromatic x-ray apparatus;

FIG. 11A illustrates a monochromatic x-ray device, in accordance withsome embodiments;

FIG. 11B illustrates a zoomed in view of components of the monochromaticx-ray device illustrated in FIG. 11A;

FIG. 11C illustrates a zoomed in view of components of the monochromaticx-ray device illustrate in FIG. 11A using a hybrid material interfaceportion, in accordance with some embodiments;

FIG. 12 illustrates a removeable carrier configured to be inserted andcapable of being removed from a receptacle of a monochromatic x-raydevice;

FIGS. 13A, 13B and 13C illustrate views of a secondary target carrier,in accordance with some embodiments;

FIGS. 14A and 14B illustrate on-axis and off-axis x-ray spectra of x-rayradiation emitted from a monochromatic x-ray apparatus using theexemplary carrier illustrated in FIGS. 13A, 13B and 13C;

FIG. 14C illustrates field of view characteristic of the x-ray spectraillustrated in FIGS. 10A-B and FIGS. 14A-14B;

FIG. 15 illustrates integrated power ratios in the low and high energyspectra as a function of viewing angle;

FIG. 16 illustrates monochromaticity as a function of viewing angle;

FIGS. 17A, 17B and 17C illustrate views of a secondary target carrier,in accordance with some embodiments;

FIGS. 18A and 18B illustrate on-axis and off-axis x-ray spectra of x-rayradiation emitted from a monochromatic x-ray apparatus using theexemplary carrier illustrated in FIGS. 17A, 17B and 17C;

FIG. 19 illustrate fluorescent x-ray spectra of secondary targets offour exemplary materials;

FIG. 20 illustrates x-ray intensity as a function of emission currentfor a number of primary voltages for secondary targets of two differentgeometries;

FIG. 21 illustrates the x-ray spectrum emitted from a gold primarytarget;

FIG. 22 illustrates on-axis and off-axis monochromaticity as a functionof primary voltage for a tin secondary target using the carrierillustrated in FIGS. 17A, 17B and 17C;

FIG. 23 illustrates on-axis and off-axis monochromaticity as a functionof primary voltage for a silver secondary target using the carrierillustrated in FIGS. 17A, 17B and 17C;

FIGS. 24A and 24B illustrate a cross-section of a monochromatic x-raysource 2400 with improved electron optics, in accordance with someembodiments;

FIG. 25 illustrate the locus of points where the electrons strike theprimary target in the monochromatic x-ray source illustrated in FIGS.24A and 24B;

FIG. 26 illustrate the locus of points where the electrons strike theprimary target in the monochromatic x-ray source illustrated in FIGS.24A and 24B.

FIG. 27 illustrates a monochromatic x-ray source including a hybridinterface component;

FIG. 28 illustrates an alternative configuration in which the cathode ismoved further away from the primary target, resulting in divergentelectron trajectories and reduced monochromaticity.

FIG. 29 illustrates a mammographic phantom used to perform imagingexperiment using monochromatic x-ray sources described herein;

FIG. 30 illustrates histograms of the embedded linear array of blocks ofthe phantom illustrated in FIG. 29;

FIG. 31 illustrates images of the phantom in FIG. 29 using a commercialbroadband x-ray system and a monochromatic x-ray system according tosome embodiments, along with corresponding histograms;

FIG. 32 illustrates stacked mammographic phantoms to model thick breasttissue;

FIG. 33 illustrates images of the phantom in FIG. 32 using a commercialbroadband x-ray system and a monochromatic x-ray system according tosome embodiments, along with corresponding histograms;

FIG. 34 illustrates conventional broadband mammography versusmonochromatic mammography according to some embodiments;

FIG. 35 illustrates images of micro-calcifications using a commercialbroadband x-ray system and a monochromatic x-ray system according tosome embodiments, along with corresponding histograms;

FIG. 36 illustrates images of micro-calcifications using a commercialbroadband x-ray system and a monochromatic x-ray system according tosome embodiments, along with corresponding histograms;

FIG. 37 illustrates line resolutions for different secondary targets anda commercial broadband x-ray system;

FIG. 38 illustrates the modulation transfer function (MTF) for themonochromatic instrument;

FIG. 39 illustrates power requirements needed for desired signal tonoise ratios for different exposure times and cone geometries;

FIG. 40 illustrates power requirements needed for desired signal tonoise ratios for different exposure times and cone geometries and withan indication of a commercial machine;

FIG. 41 illustrates the mass absorption coefficient curve for iodine.

FIG. 42 illustrates an example of contrast enhanced imaging using Ag Kx-rays at 22 keV and an iodine contrast agent called Oxilan 350.

DETAILED DESCRIPTION

As discussed above, conventional x-ray systems capable of generatingmonochromatic radiation to produce diagnostic images are typically notsuitable for clinical and/or commercial use due to the prohibitivelyhigh costs of manufacturing, operating and maintaining such systemsand/or because the system footprints are much too large for clinic andhospital use. As a result, research with these systems are limited inapplication to investigations at and by the relatively few researchinstitutions that have invested in large, complex and expensiveequipment.

Cost effective monochromatic x-ray imaging in a clinical setting hasbeen the goal of many physicists and medical professionals for decades,but medical facilities such as hospitals and clinics remain without aviable option for monochromatic x-ray equipment that can be adopted in aclinic for routine diagnostic use.

The inventor has developed methods and apparatus for producingselectable, monochromatic x-radiation over a relatively largefield-of-view (FOV). Numerous applications can benefit from such amonochromatic x-ray source, in both the medical and non-medicaldisciplines. Medical applications include, but are not limited to,imaging of breast tissue, the heart, prostate, thyroid, lung, brain,torso and limbs. Non-medical disciplines include, but are not limitedto, non-destructive materials analysis via x-ray absorption, x-raydiffraction and x-ray fluorescence. The inventor has recognized that 2Dand 3D X-ray mammography for routine breast cancer screening couldimmediately benefit from the existence of such a monochromatic source.

According to some embodiments, selectable energies (e.g., up to 100 key)are provided to optimally image different anatomical features. Someembodiments facilitate providing monochromatic x-ray radiation having anintensity that allows for relatively short exposure times, reducing theradiation dose delivered to a patient undergoing imaging. According tosome embodiments, relatively high levels of intensity can be maintainedusing relatively small compact regions from which monochromatic x-rayradiation is emitted, facilitating x-ray imaging at spatial resolutionssuitable for high quality imaging (e.g., breast imaging). The ability togenerate relatively high intensity monochromatic x-ray radiation fromrelatively small compact regions facilitates short, low dose imaging atrelatively high spatial resolution that, among other benefits, addressesone or more problems of conventional x-ray imaging systems (e.g., byovercoming difficulties in detecting cancerous lesions in thick breasttissue while still maintaining radiation dose levels below the limit setby regulatory authorities, according to some embodiments).

With conventional mammography systems, large (thick) and dense breastsare difficult, if not impossible, to examine at the same level ofconfidence as smaller, normal density breast tissue. This seriouslylimits the value of mammography for women with large and/or densebreasts (30-50% of the population), a population of women who have asix-fold higher incidence of breast cancer. The detection sensitivityfalls from 85% to 64% for women with dense breasts and to 45% for womenwith extremely dense breasts. Additionally, using conventional x-rayimaging systems (i.e., broadband x-ray imaging systems) false positivesand unnecessary biopsies occur at unsatisfactory levels. Techniquesdescribed herein facilitate monochromatic x-ray imaging capable ofproviding a better diagnostic solution for women with large and/or densebreasts who have been chronically undiagnosed, over-screened and aremost at risk for breast cancer. Though benefits associated with someembodiments have specific advantages for thick and/or dense breasts, itshould be appreciated that techniques provided herein for monochromaticx-ray imaging also provide advantages for screening of breasts of anysize and density, as well as providing benefits for other clinicaldiagnostic applications. For example, techniques described hereinfacilitate reducing patient radiation dose by a factor of 6-26 dependingon tissue density for all patients over conventional x-ray imagingsystems currently deployed in clinical settings, allowing for annual andrepeat exams while significantly reducing the lifetime radiationexposure of the patient. Additionally, according to some embodiments,screening may be performed without painful compression of the breast incertain circumstances. Moreover, the technology described hereinfacilitates the manufacture of monochromatic x-ray systems that arerelatively low cost, keeping within current cost constraints ofbroadband x-ray systems currently in use for clinical mammography.

Monochromatic x-ray imaging may be performed with approved contrastagents to further enhance detection of tissue anomalies at a reduceddose. Techniques described herein may be used with three dimensional 3Dtomosynthesis at similarly low doses. Monochromatic radiation usingtechniques described herein may also be used to perform in-situ chemicalanalysis (e.g., in-situ analysis of the chemical composition of tumors),for example, to improve the chemical analysis techniques described inU.S. patent application Ser. No. 15/825,787, filed Nov. 28, 2017 andtitled “Methods and Apparatus for Determining Information RegardingChemical Composition Using X-ray Radiation,” which application isincorporated herein in its entirety.

Conventional monochromatic x-ray sources have previously been developedfor purposes other than medical imaging and, as a result, are generallyunsuitable for clinical purposes. Specifically, the monochromaticity,intensity, spatial resolution and/or power levels may be insufficientfor medical imaging purposes. The inventor has developed techniques forproducing monochromatic x-ray radiation suitable for numerousapplications, including for clinical purposes such as breast and othertissue imaging, aspects of which are described in further detail below.The inventor recognized that conventional monochromatic x-ray sourcesemit significant amounts of broadband x-ray radiation in addition to theemitted monochromatic x-ray radiation. As a result, the x-ray radiationemitted from such monochromatic x-ray sources have poor monochromaticitydue to the significant amounts of broadband radiation that is alsoemitted by the source, contaminating the x-ray spectrum.

The inventor has developed techniques for producing x-ray radiation withhigh degrees of monochromaticity (e.g., as measured by the ratio ofmonochromatic x-ray radiation to broadband radiation as discussed infurther detail below), both in the on-axis direction and off-axisdirections over a relatively large field of view. Techniques describedherein enable the ability to increase the power of the broadband x-raysource without significantly increasing broadband x-ray radiationcontamination (i.e., without substantially reducing monochromaticity).As a result, higher intensity monochromatic x-ray radiation may beproduced using increased power levels while maintaining high degrees ofmonochromaticity.

According to some embodiments, a monochromatic x-ray device is providedthat is capable of producing monochromatic x-ray radiation havingcharacteristics (e.g., monochromaticity, intensity, etc.) that enableexposure times of less than 20 seconds and, according to someembodiments, exposure times of less than 10 seconds for mammography.

According to some embodiments, a monochromatic x-ray device is providedthat emits monochromatic x-rays having a high degree of monochromaticity(e.g., at 90% purity or better) over a field of view sufficient to imagea target organ (e.g., a breast) in a single exposure to produce an imageat a spatial resolution suitable for diagnostics (e.g., a spatialresolution of a 100 microns or better).

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, monochromatic x-ray systems andtechniques regarding same. It should be appreciated that the embodimentsdescribed herein may be implemented in any of numerous ways. Examples ofspecific implementations are provided below for illustrative purposesonly. It should be appreciated that the embodiments and thefeatures/capabilities provided may be used individually, all together,or in any combination of two or more, as aspects of the technologydescribed herein are not limited in this respect.

FIG. 9 illustrates a two dimensional (2D) schematic cut of aconventional x-ray apparatus for generating monochromatic x-rays viax-ray fluoresence. The x-ray apparatus illustrated in FIG. 9 is similarin geometry to the x-ray apparatus illustrated and described in U.S.Pat. No. 4,903,287, titled “Radiation Source for Generating EssentiallyMonochromatic X-rays,” as well as the monochromatic x-ray sourceillustrated and described in Marfeld, et al., Proc. SPIE Vol. 4502, p.117-125, Advances in Laboratory-based X-ray Sources and Optics II, AliM. Khounsayr; Carolyn A. MacDonald; Eds. Referring to FIG. 9, x-rayapparatus 900 comprises a vacuum tube 950 that contains a toroidalfilament 905 that operates as a cathode and primary target 910 thatoperates as an anode of the circuit for generating broadband x-rayradiation. Vacuum tube 950 includes a vacuum sealed enclosure formedgenerally by housing 955, front portion 965 (e.g., a copper faceplate)and a window 930 (e.g., a beryllium window).

In operation, electrons (e.g., exemplary electrons 907) from filament905 (cathode) are accelerated toward primary target 910 (anode) due tothe electric field established by a high voltage bias between thecathode and the anode. As the electrons are decelerated by the primarytarget 910, broadband x-ray radiation 915 (i.e., Bremsstrahlungradiation as shown in FIG. 3) is produced. Characteristic emission linesunique to the primary target material may also be produced by theelectron bombardment of the anode material provided the voltage is largeenough to produce photoelectrons. Thus, broadband x-ray radiation (oralternatively broad spectrum radiation) refers to Bremsstrahlungradiation with or without characteristic emission lines of the primarytarget. The broadband radiation 915 emitted from primary target 910 istransmitted through window 930 of the vacuum enclosure to irradiatesecondary target 920. Window 930 provides a transmissive portion of thevacuum enclosure made of a material (e.g., beryllium) that generallytransmits broadband x-ray radiation generated by primary target 910 andblocks electrons from impinging on the secondary target 920 (e.g.,electrons that scatter off of the primary target) to prevent unwantedBremststralung radiation from being produced. Window 930 may becup-shaped to accommodate secondary target 920 outside the vacuumenclosure, allowing the secondary target to be removed and replacedwithout breaking the vacuum seal of x-ray tube 950.

In response to incident broadband x-ray radiation from primary target910, secondary target 920 generates, via fluorescence, monochromaticx-ray radiation 925 characteristic of the element(s) in the secondtarget. Secondary target 920 is conical in shape and made from amaterial selected so as to produce fluorescent monochromatic x-rayradiation at a desired energy, as discuss in further detail below.Broadband x-ray radiation 915 and monochromatic x-ray radiation 925 areillustrated schematically in FIG. 9 to illustrate the general principleof using a primary target and a secondary target to generatemonochromatic x-ray radiation via fluorescence. It should be appreciatedthat broadband and monochromatic x-ray radiation will be emitted in the4π directions by the primary and secondary targets, respectively.Accordingly, x-ray radiation will be emitted from x-ray tube 950 atdifferent angles θ relative to axis 955 corresponding to thelongitudinal axis through the center of the aperture of x-ray tube 950.

As discussed above, the inventor has recognized that conventional x-rayapparatus for generating monochromatic x-ray radiation (also referred toherein as monochromatic x-ray sources) emit significant amounts ofbroadband x-ray radiation. That is, though conventional monochromaticsources report the ability to produce monochromatic x-ray radiation, inpractice, the monochromaticity of the x-ray radiation emitted by theseconventional apparatus is poor (i.e., conventional monochromatic sourcesexhibit low degrees of monochromaticity. For example, the conventionalmonochromatic source described in Marfeld, using a source operated at165 kV with a secondary target of tungsten (W), emits monochromaticx-ray radiation that is approximately 50% pure (i.e., the x-ray emissionis approximately 50% broadband x-ray radiation). As another example, aconventional monochromatic x-ray source of the general geometryillustrated in FIG. 9, operating with a cathode at a negative voltage of−50 kV, a primary target made of gold (Au; Z=79) at ground potential,and a secondary target made of tin (Sn; Z=50), emits the x-ray spectraillustrated in FIG. 10A (on-axis) and FIG. 10B (off-axis). As discussedabove, x-ray radiation will be emitted from the x-ray tube at differentangles θ relative to the longitudinal axis of the x-ray tube (axis 955illustrated in FIG. 9).

Because the on-axis spectrum and the off-axis spectrum play a role inthe efficacy of a monochromatic source, both on-axis and off-axis x-rayspectra are shown. In particular, variation in the monochromaticity ofx-ray radiation as a function of the viewing angle θ results innon-uniformity in the resulting images. In addition, for medical imagingapplications, decreases in monochromaticity (i.e., increases in therelative amount of broadband x-ray radiation) of the x-ray spectra atoff-axis angles increases the dose delivered to the patient. Thus, thedegree of monochromaticity of both on-axis and off-axis spectra may bean important property of the x-ray emission of an x-ray apparatus. InFIG. 10A, on-axis refers to a narrow range of angles about the axis ofthe x-ray tube (less than approximately 0.5 degrees), and off-axisrefers to approximately 5 degrees off the axis of the x-ray tube. Asshown in FIGS. 10A and 10B, the x-ray spectrum emitted from theconventional monochromatic x-ray source is not in fact monochromatic andis contaminated with significant amounts of broadband x-ray radiation.

In particular, in addition to the characteristic emission lines of thesecondary target (i.e., the monochromatic x-rays emitted via K-shellfluorescence from the tin (Sn) secondary target resulting fromtransitions from the L and M-shells, labeled as Sn K_(α) and Sn K_(β) inFIGS. 10A and 10B, respectively), x-ray spectra 1000 a and 1000 b shownin FIGS. 10A and 10B also include significant amounts of broadband x-rayradiation. Specifically, x-ray spectra 1000 a and 1000 b includesignificant peaks at the characteristic emission lines of the primarytarget (i.e., x-ray radiation at the energies corresponding to K-shellemissions of the gold primary target, labeled as Au Kα and Au Kβ inFIGS. 10A and 10B), as well as significant amounts of Bremsstrahlungbackground. As indicated by arrows 1003 in FIGS. 10A and 10B, the SnK_(α) peak is only (approximately) 8.7 times greater than theBremsstrahlung background in the on-axis direction and approximately 7times greater than the Bremsstrahlung background in the off-axisdirection. Thus, it is clear from inspection alone that thisconventional monochromatic x-ray source emits x-ray radiation exhibitingstrikingly poor monochromaticity, both on and off-axis, as quantifiedbelow.

Monochromaticity may be computed based on the ratio of the integratedenergy in the characteristic fluorescent emission lines of the secondarytarget to the total integrated energy of the broadband x-ray radiation.For example, the integrated energy of the low energy broadband x-rayradiation (e.g., the integrated energy of the x-ray spectrum below theSn K_(a) peak indicated generally by arrows 1001 in FIGS. 10A and 10B),referred to herein as P_(low), and the integrated energy of the highenergy broadband x-ray radiation (e.g., the integrated energy of thex-ray spectrum above the Sn K_(β) peak indicated generally by arrows1002 in FIGS. 10A and 10B), referred to herein as P_(high), may becomputed. The ratio of the integrated energy of the characteristicK-shell emission lines (referred to herein as P_(k), which correspondsto the integrated energy in the Sn K_(α) and the Sn K_(β) emissions inFIGS. 10A and 10B) to P_(low), and P_(high) provides a measure of theamount of broadband x-ray radiation relative to the amount ofmonochromatic x-ray radiation emitted by the x-ray source. In theexample of FIG. 10A, the ratio P_(k)/P_(low), is 0.69 and the ratioP_(k)/P_(high) is 1.7. In the example of FIG. 10B, the ratioP_(k)/P_(low) is 0.9 and the ratio P_(k)/P_(high) is 2.4. Increasing theratios P_(low) and P_(high) increases the degree to which the spectraloutput of the source is monochromatic. As used herein, themonochromaticity, M, of an x-ray spectrum is computed asM=1/(1+1/a+1/b), where a=P_(k)/P_(low), b=P P_(k)/P_(high). For theon-axis x-ray spectrum in FIG. 10A produced by the conventional x-rayapparatus, M=0.33, and for the off-axis x-ray spectrum in FIG. 10Bproduced by the conventional x-ray apparatus, M=0.4. As such, themajority of the energy of the x-ray spectrum is broadband x-rayradiation and not monochromatic x-ray radiation.

The inventor has developed techniques that facilitate generating anx-ray radiation having significantly higher monochromaticity, thusimproving characteristics of the x-ray emission from an x-ray device andfacilitating improved x-ray imaging. FIG. 11A illustrates an x-raydevice 1100 incorporating techniques developed by the inventor toimprove properties of the x-ray radiation emitted from the device, andFIG. 11B illustrates a zoomed in view of components of the x-ray device1100, in accordance with some embodiments. X-ray device 1100 comprises avacuum tube 1150 providing a vacuum sealed enclosure for electron optics1105 and primary target 1110 of the x-ray device. The vacuum sealedenclosure is formed substantially by a housing 1160 (which includes afront portion 1165) and an interface or window portion 1130. Faceplate1175 may be provided to form an outside surface of front portion 1165.Faceplate 1175 may be comprised of material that is generally opaque tobroadband x-ray radiation, for example, a high Z material such as lead,tungsten, thick stainless steel, tantalum, rhenium, etc. that preventsat least some broadband x-ray radiation from being emitted from x-raydevice 1100.

Interface portion 1130 may be comprised of a generally x-raytransmissive material (e.g., beryllium) to allow broadband x-rayradiation from primary target 1110 to pass outside the vacuum enclosureto irradiate secondary target 1120. In this manner, interface portion1130 provides a “window” between the inside and outside the vacuumenclosure through which broadband x-ray radiation may be transmittedand, as result, is also referred to herein as the window or windowportion 1130. Window portion 1130 may comprise an inner surface facingthe inside of the vacuum enclosure and an outer surface facing theoutside of the vacuum enclosure of vacuum tube 1150 (e.g., inner surface1232 and outer surface 1234 illustrated in FIG. 12). Window portion 1130may be shaped to form a receptacle (see receptacle 1235 labeled in FIG.12) configured to hold secondary target carrier 1140 so that thesecondary target (e.g., secondary target 1120) is positioned outside thevacuum enclosure at a location where at least some broadband x-rayradiation emitted from primary target 1110 will impinge on the secondarytarget. According to some embodiments, carrier 1140 is removable. Byutilizing a removable carrier 1140, different secondary targets can beused with x-ray system 1100 without needing to break the vacuum seal, asdiscussed in further detail below. However, according to someembodiments, carrier 1140 is not removable.

The inventor recognized that providing a hybrid interface portioncomprising a transmissive portion and a blocking portion facilitatesfurther reducing the amount of broadband x-ray radiation emitted fromthe x-ray device. For example, FIG. 11C illustrates an interface portion1130′ comprising a transmissive portion 1130 a (e.g., a berylliumportion) and a blocking portion 1130 b (e.g., a tungsten portion), inaccordance with some embodiments. Thus, according to some embodiments,interface portion 1130′ may comprise a first material below the dashedline in FIG. 11C and comprise a second material different from the firstmaterial above the dashed line. Transmissive portion 1130 a and blockingportion 1130 b may comprise any respective material suitable forperforming intended transmission and absorption function sufficiently,as the aspect are not limited for use with any particular materials.

According to some embodiments, the location of the interface between thetransmissive portion and the blocking portion (e.g., the location of thedashed line in FIG. 11C) approximately corresponds to the location ofthe interface between the transmissive portion and the blocking portionof the carrier when the carrier is inserted into the receptacle formedby the interface portion. According to some embodiments, the location ofthe interface between the transmissive portion and the blocking portion(e.g., the location of the dashed line in FIG. 11C) does not correspondto the location of the interface between the transmissive portion andthe blocking portion of the carrier when the carrier is inserted intothe receptacle formed by the interface portion. A hybrid interfacecomponent is also illustrated in FIG. 28A, discussed in further detailbelow.

In the embodiment illustrated in FIGS. 11A and 11B, secondary target1120 has a conical geometry and is made of a material that fluorescesx-rays at desired energies in response to incident broadband x-rayradiation. Secondary target may be made of any suitable material,examples of which include, but are not limited to tin (Sn), silver (Ag),molybdenum (Mo), palladium (Pd), or any other suitable material orcombination of materials. FIG. 19 illustrates the x-ray spectraresulting from irradiating secondary target cones of the four exemplarymaterials listed above. Secondary target 1120 provides a small compactregion from which monochromatic x-ray radiation can be emitted viafluorescent to provide good spatial resolution, as discussed in furtherdetail below.

The inventor has appreciated that removable carrier 1140 can be designedto improve characteristics of the x-ray radiation emitted from vacuumtube 1150 (e.g., to improve the monochromaticity of the x-ray radiationemission). Techniques that improve the monochromaticity also facilitatethe ability to generate higher intensity monochromatic x-ray radiation,as discussed in further detail below. In the embodiment illustrated inFIGS. 11A and 11B, removable carrier 1140 comprises a transmissiveportion 1142 that includes material that is generally transmissive tox-ray radiation so that at least some broadband x-ray radiation emittedby primary target 1110 that passes through window portion 1130 alsopasses through transmissive portion 1142 to irradiate secondary target1120. Transmissive portion 1142 may include a cylindrical portion 1142 aconfigured to accommodate secondary target 1120 and may be configured toallow the secondary target to be removed and replaced so that secondarytargets of different materials can be used to generate monochromaticx-rays at the different characteristic energies of the respectivematerial, though the aspects are not limited for use with a carrier thatallows secondary targets to be interchanged (i.e., removed andreplaced). Exemplary materials suitable for transmissive portion 1142include, but are not limited to, aluminum, carbon, carbon fiber, boron,boron nitride, beryllium oxide, silicon, silicon nitride, etc.

Carrier 1140 further comprises a blocking portion 1144 that includesmaterial that is generally opaque to x-ray radiation (i.e., materialthat substantially absorbs incident x-ray radiation). Blocking portion1144 is configured to absorb at least some of the broadband x-rayradiation that passes through window 1130 that is not converted byand/or is not incident on the secondary target and/or is configured toabsorb at least some of the broadband x-ray radiation that mightotherwise escape the vacuum enclosure. In conventional x-rays sources(e.g., conventional x-ray apparatus 900 illustrated in FIG. 9),significant amounts of broadband x-ray radiation is allowed to beemitted from the apparatus, corrupting the fluorescent x-ray radiationemitted by the secondary target and substantially reducing themonochromaticity of the emitted x-ray radiation. In the embodimentsillustrated in FIGS. 11A, 11B, 12, 13A-C and 17A-C, the transmissiveportion and the blocking portion form a housing configured toaccommodate the secondary target.

According to some embodiments, blocking portion 1144 includes acylindrical portion 1144 a and an annular portion 1144 b. Cylindricalportion 1144 a allows x-ray radiation fluoresced by the secondary target1120 in response to incident broadband x-ray radiation from primarytarget 1110 to be transmitted, while absorbing at least some broadbandx-ray radiation as discussed above. Annular portion 1144 b provides aportion providing increased surface area to absorb additional broadbandx-ray radiation that would otherwise be emitted by the x-ray device1100. In the embodiment illustrated in FIGS. 11A and 11B, annularportion 1144 b is configured to fit snugly within a recess in the frontportion of the x-ray tube to generally maximize the amount of broadbandx-ray radiation that is absorbed to the extent possible. Annular portion1144 b includes an aperture portion 1144 c that corresponds to theaperture through cylindrical portions 1144 b and 1142 a to allowmonochromatic x-ray radiation fluoresced from secondary target 1120 tobe emitted from x-ray device 1100, as also shown in FIGS. 13B and 17Bdiscussed below. Exemplary materials suitable for blocking portion 1144include, but are not limited to, lead, tungsten, tantalum, rhenium,platinum, gold, etc.

In the embodiment illustrated FIGS. 11A and 11B, carrier 1140 isconfigured so that a portion of the secondary target is contained withinblocking portion 1144. Specifically, as illustrated in the embodimentshown in FIGS. 11A and 11B, the tip of conical secondary target 1120extends into cylindrical portion 1144 b when the secondary target isinserted into transmissive portion 1142 of carrier 1140. The inventorhas appreciated that having a portion of the secondary target containedwithin blocking portion 1144 improves characteristics of themonochromatic x-ray radiation emitted from the x-ray device, asdiscussed in further below. However, according to some embodiments, asecondary target carrier may be configured so that no portion of thesecondary target is contained with the blocking portion of the carrier,examples of which are illustrated FIGS. 13A-C discussed in furtherdetail below. Both configurations of carrier 1140 (e.g., with andwithout blocking overlap of the secondary target carrier) providesignificant improvements to characteristics of the emitted x-rayradiation (e.g., improved monochromaticity), as discussed in furtherdetail below.

As illustrated in FIG. 12, carrier 1240 (which may be similar or thesame as carrier 1140 illustrated in FIGS. 11A and 11B) is configured tobe removeable. For example, carrier 1240 may be removeably inserted intoreceptacle 1235 formed by interface component 1230 (e.g., an interfacecomprising a transmissive window), for example, by inserting andremoving the carrier, respectively, in the directions generallyindicated by arrow 1205. That is, according to some embodiments, carrier1240 is configured as a separate component that can be inserted into andremoved from the x-ray device (e.g., by inserting removeable carrier1240 into and/or removing the carrier from receptacle 1235).

As shown in FIG. 12, carrier 1240 has a proximal end 1245 configured tobe inserted into the x-ray device and a distal end 1247 from whichmonochromatic x-ray radiation is emitted via aperture 1244 d through thecenter of carrier 1240. In the embodiment illustrated in FIG. 12,cylindrical blocking portion 1244 a is positioned adjacent to anddistally from cylindrical transmissive portion 1242 a. Annular blockingportion 1244 b is positioned adjacent to and distally from block portion1244 a. As shown, annular blocking portion 1244 b has a diameter D thatis larger than a diameter d of the cylindrical blocking portion 1244 a(and cylindrical transmissive portion 1242 a for embodiments in whichthe two cylindrical portions have approximately the same diameter). Thedistance from the extremes of the proximal end and the distal end islabeled as height H in FIG. 12. The dimensions of carrier 1240 maydepend on the dimensions of the secondary target that the carrier isconfigured to accommodate. For example, for an exemplary carrier 1240configured to accommodate a secondary target having a 4 mm base,diameter d may be approximately 4-5 mm, diameter D may be approximately13-16 mm, and height H may be approximately 18-22 mm. As anotherexample, for an exemplary carrier 1240 configured to accommodate asecondary target having a 8 mm base, diameter d may be approximately 8-9mm, diameter D may be approximately 18-22 mm, and height H may beapproximately 28-32 mm. It should be appreciated that the dimensions forthe carrier and the secondary target provided are merely exemplary, andcan be any suitable value as the aspect are not limited for use with anyparticular dimension or set of dimensions.

According to some embodiments, carrier 1240 may be configured to screwinto receptacle 1235, for example, by providing threads on carrier 1240capable of being hand screwed into cooperating threads within receptacle1235. Alternatively, a releasable mechanical catch may be provided toallow the carrier 1240 to be held in place and allows the carrier 1240to be removed by applying force outward from the receptacle. As anotheralternative, the closeness of the fit of carrier 1240 and receptacle1235 may be sufficient to hold the carrier in place during operation.For example, friction between the sides of carrier 1240 and the walls ofreceptacle 1235 may be sufficient to hold carrier 1240 in position sothat no additional fastening mechanism is needed. It should beappreciated that any means sufficient to hold carrier 1240 in positionwhen the carrier is inserted into the receptacle may be used, as theaspects are not limited in this respect.

As discussed above, the inventor has developed a number of carrierconfiguration that facilitate improved monochromatic x-ray radiationemission. FIGS. 13A and 13B illustrate a three-dimensional and atwo-dimensional view of a carrier 1340, in accordance with someembodiments. The three-dimensional view in FIG. 13A illustrates carrier1340 separated into exemplary constituent parts. In particular, FIG. 13Aillustrates a transmissive portion 1342 separated from a blockingportion 1344. As discussed above, transmissive portion 1342 may includematerial that generally transmits broadband x-ray radiation at least atthe relevant energies of interest (i.e., material that allows broadbandx-ray radiation to pass through the material without substantialabsorption at least at the relevant energies of interest, such asaluminum, carbon, carbon fiber, boron, boron nitride, beryllium oxide,silicon, silicon nitride, etc. Blocking portion 1344, on the other hand,may include material that is generally opaque to broadband x-rayradiation at least at the relevant energies of interest (i.e., materialthat substantially absorbs broadband x-ray radiation at least at therelevant energies of interest, such as lead, tungsten, tantalum,rhenium, platinum, gold, etc.

In this way, at least some broadband x-ray radiation emitted by theprimary target is allowed to pass through transmissive portion 1342 toirradiate the secondary target, while at least some broadband x-rayradiation emitted from the primary target (and/or emitted from orscattered by other surfaces of the x-ray tube) is absorbed by blockingportion 1344 to prevent unwanted broadband x-ray radiation from beingemitted from the x-ray device. As a result, carrier 1340 facilitatesproviding monochromatic x-ray radiation with reduced contamination bybroadband x-ray radiation, significantly improving monochromaticity ofthe x-ray emission of the x-ray device. In the embodiments illustratedin FIGS. 13A-C, blocking portion 1344 includes a cylindrical portion1344 a and annular portion 1344 b having a diameter greater thancylindrical portion 1344 a to absorb broadband x-ray radiation emittedover a wider range of angles and/or originating from a wider range oflocations to improve the monochromaticity of the x-ray radiationemission of the x-ray device.

According to some embodiments, transmissive portion 1342 and blockingportion 1344 may be configured to couple together or mate using any of avariety of techniques. For example, the transmissive portion 1342,illustrated in the embodiment of FIG. 13A as a cylindrical segment, mayinclude a mating portion 1343 a at one end of the cylindrical segmentconfigured to mate with mating portion 1342 b at a corresponding end ofcylindrical portion 1344 a of blocking portion 1344. Mating portion 1343a and 1343 b may be sized appropriately and, for example, provided withthreads to allow the transmissive portion 1342 and the blocking portion1344 to be mated by screwing the two portion together. Alternatively,mating portion 1343 a and 1343 b may be sized so that mating portion1343 a slides over mating portion 1343 b, or vice versa, to couple thetwo portions together. It should be appreciated that any mechanism maybe used to allow transmissive portion 1342 and blocking portion 1344 tobe separated and coupled together. According to some embodiments,transmissive portion 1342 and blocking portion 1344 are not separable.For example, according to some embodiments, carrier 1340 may bemanufactured as a single component having transmissive portion 1342fixedly coupled to blocking portion 1344 so that the portions are notgenerally separable from one another as a general matter of course.

Transmissive portion 1342 may also include portion 1325 configured toaccommodate secondary target 1320. For example, one end of transmissiveportion 1342 may be open and sized appropriately so that secondarytarget 1320 can be positioned within transmissive portion 1342 so that,when carrier 1340 is coupled to the x-ray device (e.g., inserted into areceptacle formed by an interface portion of the vacuum tube, such as atransmissive window or the like), secondary target 1320 is positioned sothat at least some broadband x-ray radiation emitted from the primarytarget irradiates secondary target 1320 to cause secondary target tofluoresce monochromatic x-rays at the characteristic energies of theselected material. In this way, different secondary targets 1320 can bepositioned within and/or held by carrier 1340 so that the energy of themonochromatic x-ray radiation is selectable. According to someembodiments, secondary target 1320 may include a portion 1322 thatfacilitates mating or otherwise coupling secondary target 1320 to thecarrier 1340. For example, portions 1322 and 1325 may be provide withcooperating threads that allow the secondary target to be screwed intoplace within the transmissive portion 1342 of carrier 1340.Alternatively, portions 1322 and 1325 may be sized so that the secondarytarget fits snuggly within transmissive portion and is held by thecloseness of the fit (e.g., by the friction between the two components)and/or portion 1322 and/or portion 1325 may include a mechanical featurethat allows the secondary target to held into place. According to someembodiments, a separate cap piece may be included to fit overtransmissive portion 1342 after the secondary target has been insertedinto the carrier and/or any other suitable technique may be used toallow secondary target 1320 to be inserted within and sufficiently heldby carrier 1340, as the aspects are not limited in this respect.

In the embodiment illustrated in FIG. 13B, secondary target 1320 iscontained within transmissive portion 1342, without overlap withblocking portion 1344. That is, the furthest extent of secondary target1320 (e.g., the tip of the conical target in the embodiment illustratedin FIG. 13B) does not extend into cylindrical portion 1344 a of theblocking portion (or any other part of the blocking portion). Bycontaining secondary target 1320 exclusively within the transmissiveportion of the carrier, the volume of secondary target 1320 exposed tobroadband x-ray radiation and thus capable of fluorescing monochromaticx-ray radiation may be generally maximized, providing the opportunity togenerally optimize the intensity of the monochromatic x-ray radiationproduced for a given secondary target and a given set of operatingparameters of the x-ray device (e.g., power levels of the x-ray tube,etc.). That is, by increasing the exposed volume of the secondarytarget, increased monochromatic x-ray intensity may be achieved.

The front view of annular portion 1344 b of blocking portion 1334illustrated in FIG. 13B illustrates that annular portion 1344 b includesaperture 1344 c corresponding to the aperture of cylindrical portion1344 a (and cylindrical portion 1342) that allows monochromatic x-raysfluoresced from secondary target 1320 to be emitted from the x-raydevice. Because blocking portion 1344 is made from a generally opaquematerial, blocking portion 1344 will also absorb some monochromaticx-rays fluoresced from the secondary target emitted at off-axis anglesgreater than some threshold angle, which threshold angle depends onwhere in the volume of the secondary target the monochromatic x-raysoriginated. As such, blocking portion 1344 also operates as a collimatorto limit the monochromatic x-rays emitted to a range of angles relativeto the axis of the x-ray tube, which in the embodiments in FIGS. 13A-C,corresponds to the longitudinal axis through the center of carrier 1340.

FIG. 13C illustrates a schematic of carrier 1340 positioned within anx-ray device (e.g., inserted into a receptacle formed by an interfaceportion of the vacuum tube, such as exemplary window portions 1130 and1230 illustrated in FIGS. 11A, 11B and 12). Portions 1365 correspond tothe front portion of the vacuum tube, conventionally constructed of amaterial such as copper. In addition, a cover or faceplate 1375 made ofa generally opaque material (e.g., lead, tungsten, tantalum, rhenium,platinum, gold, etc.) is provided having an aperture corresponding tothe aperture of carrier 1340. Faceplate 1375 may be optionally includedto provide further absorption of broadband x-ray to prevent spuriousbroadband x-ray radiation from contaminating the x-ray radiation emittedfrom the x-ray device.

According to some embodiments, exemplary carrier 1340 may be used toimprove monochromatic x-ray emission characteristics. For example, FIGS.14A and 14B illustrate the on-axis x-ray spectrum 1400 a and off-axisx-ray spectrum 1400 b resulting from the use of carrier 1340 illustratedin FIGS. 13A, 13B and/or 13C. As shown, the resulting x-ray spectrum issignificantly improved relative to the on-axis and off-axis x-rayspectra shown in FIGS. 10A and 10B that was produced by a conventionalx-ray apparatus configured to produce monochromatic x-ray radiation(e.g., conventional x-ray apparatus 900 illustrated in FIG. 9). Asindicated by arrow 1403 in FIG. 14A, the on-axis Sn K_(a) peak isapproximately 145 times greater than the Bremsstrahlung background, upfrom approximately 8.7 in the on-axis spectrum illustrated in FIG. 10A.The off-axis Sn K_(a) peak is approximately 36 times greater than theBremsstrahlung background as indicated by arrow 1403 in FIG. 14B, upfrom approximately 7.0 in the off-axis spectrum illustrated in FIG. 14B.In addition, the ratios of P_(k) (the integrated energy of thecharacteristic K-shell emission lines, labeled as Sn K_(α) and Sn K_(β)in FIGS. 14A and 14B) to P_(low) (the integrated energy of the lowenergy x-ray spectrum below the Sn K_(α) peak, indicated generally byarrows 1401 in FIGS. 14A and 14B) and P_(high) (the integrated energy ofthe high energy spectrum above the Sn K_(β) peak, indicated generally byarrows 1402) are 21 and 62, respectively, for the on-axis spectrumillustrated in FIG. 14A, up from 0.69 and 1.7 for the on-axis spectrumof FIG. 10A. The ratios P_(k)/P_(low) and P_(k)/P_(high) are 12.9 and22, respectively, for the off-axis spectrum illustrated in FIG. 14B, upfrom 0.9 and 2.4 for the off-axis spectrum of FIG. 10B. These increasedratios translate to an on-axis monochromaticity of 0.94 (M=0.94) and anoff-axis monochromaticity of 0.89 (M=0.89), up from an on-axismonochromaticity of 0.33 and an off-axis monochromaticity of 0.4 for thex-ray spectrum of FIGS. 10A and 10B, respectively.

This significant improvement in monochromaticity facilitates acquiringx-ray images that are more uniform, have better spatial resolution andthat deliver significantly less x-ray radiation dose to the patient inmedical imaging applications. For example, in the case of mammography,the x-ray radiation spectrum illustrated in FIGS. 10A and 10B woulddeliver four times the mean glandular dose to normal thickness anddensity breast tissue than would be delivered by the x-ray radiationspectrum illustrated in FIGS. 14A and 14B. FIG. 14C illustrates thefield of view of the conventional x-ray source used to generate thex-ray spectrum illustrated in FIGS. 10A and 10B along with the field ofview of the x-ray device used to generate the x-ray spectrum illustratedin FIGS. 14A and 14B. The full width at half maximum (FWHM) of theconventional x-ray apparatus is approximately 30 degrees, while the FWHMof the improved x-ray device is approximately 15 degrees. Accordingly,although the field of view is reduced via exemplary carrier 1340, theresulting field of view is more than sufficient to image an organ suchas the breast in a single exposure at compact source detector distances(e.g., approximately 760 mm), but with increased uniformity and spatialresolution and decreased radiation dose, allowing for significantlyimproved and safer x-ray imaging. FIG. 15 illustrates the integratedpower ratios for the low and high energy x-ray radiation (P_(k)/P_(low)and P_(k)/P_(high)) as a function of the viewing angle θ and FIG. 16illustrates the monochromaticity of the x-ray radiation for theconventional x-ray apparatus (1560 a, 1560 b and 1660) and the improvedx-ray apparatus using exemplary carrier 1340 (1570 a, 1570 b and 1670).As shown by plots 1570 a, 1570 b and 1670, monochromaticity decreases asa function of viewing angle. Using carrier 1340, monochromatic x-rayradiation is emitted having a monochromaticity of at least 0.7 across a15 degree field of view and a monochromaticity of at least 0.8 across a10 degree field of view about the longitudinal axis. As shown by plots1560 a, 1560 b and 1660, monochromaticity of the conventional x-rayapparatus is extremely poor across all viewing angles (i.e., less than0.4 across the entire field of view).

The inventor has appreciated that further improvements to aspects of themonochromaticity of x-ray radiation emitted from an x-ray tube may beimproved by modifying the geometry of the secondary target carrier.According to some embodiments, monochromaticity may be dramaticallyimproved, in particular, for off-axis x-ray radiation. For example, theinventor recognized that by modifying the carrier so that a portion ofthe secondary target is within a blocking portion of the carrier, themonochromaticity of x-ray radiation emitted by an x-ray device may beimproved, particularly with respect to off-axis x-ray radiation. FIGS.17A and 17B illustrate a three-dimensional and a two-dimensional view ofa carrier 1740, in accordance with some embodiments. Exemplary carrier1740 may include similar parts to carrier 1340, including a transmissiveportion 1742 to accommodate secondary target 1720, and a blockingportion 1744 (which may include a cylindrical portion 1744 a and annularportion 1744 b with an aperture 1744 c through the center), as shown inFIG. 17A.

However, in the embodiment illustrated in FIGS. 17A-C, carrier 1740 isconfigured so that, when secondary target 1720 is positioned withintransmissive portion 1742, a portion of secondary target 1720 extendsinto blocking portion 1744. In particular, blocking portion includes anoverlap portion 1744 d that overlaps part of secondary target 1720 sothat at least some of the secondary target is contained within blockingportion 1744. According to some embodiments, overlap portion 1744 dextends over between approximately 0.5 and 5 mm of the secondary target.According to some embodiments, overlap portion 1744 d extends overbetween approximately 1 and 3 mm of the secondary target. According tosome embodiments, overlap portion 1744 d extends over approximately 2 mmof the secondary target. According to some embodiments, overlap portion1744 d extends over less than 0.5 mm, and in some embodiments, overlapportion 1744 d extends over greater than 5 mm. The amount of overlapwill depend in part on the size and geometry of the secondary target,the carrier and the x-ray device. FIG. 17C illustrates carrier 1740positioned within an x-ray device (e.g., inserted in a receptacle formedat the interface of the vacuum tube), with a faceplate 1775 providedover front portion 1765 of a vacuum tube (e.g., vacuum tube 1150illustrated in FIG. 11A).

According to some embodiments, exemplary carrier 1740 may be used tofurther improve monochromatic x-ray emission characteristics. Forexample, FIGS. 18A and 18B illustrate the on-axis x-ray spectrum 1800 aand off-axis x-ray spectrum 1800 b resulting from the use of carrier1740 illustrated in FIGS. 17A-C. As shown, the resulting x-ray spectrumare significantly improved relative to the on-axis and off-axis x-rayspectrum produced the conventional x-ray apparatus shown in FIGS. 10Aand 10B, as well as exhibiting improved characteristics relative to thex-ray spectra produced using exemplary carrier 1340 illustrated in FIGS.13A-C. As indicated by arrow 1803 in FIG. 18A, the on-axis Sn K_(α) peakis 160 times greater than the Bremsstrahlung background, compared to 145for the on-axis spectrum in FIG. 14A and 8.7 for the on-axis spectrumillustrated in FIG. 10A. As indicated by arrow 1803 in FIG. 18B, theoff-axis Sn K_(α) peak is 84 times greater than the Bremsstrahlungbackground, compared to 36 for the off-axis spectrum in FIG. 14B and 7.0for the off-axis spectrum illustrated in FIG. 10B.

The ratios of P_(k) (the integrated energy of the characteristic K-shellemission lines, labeled as Sn K_(α) and Sn K_(β) in FIGS. 18A and 18B)to P_(low) (the integrated energy of the low energy x-ray spectrum belowthe Sn K_(α) peak, indicated generally by arrows 1801 in FIGS. 18A and18B) and P_(high) (the integrated energy of the high energy spectrumabove the Sn K_(β) peak, indicated generally by arrows 1802) are 31 and68, respectively, for the on-axis spectrum illustrated in FIG. 18A,compared to 21 and 62 for the on-axis spectrum of FIG. 14A and 0.69 and1.7 for the on-axis spectrum of FIG. 10A. The ratios P_(k)/P_(low) andP_(k)/P_(high) are 29 and 68, respectively, for the off-axis spectrum ofFIG. 18B, compared to 12.9 and 22, respectively, for the off-axisspectrum illustrated in FIG. 14B and 0.9 and 2.4 for the off-axisspectrum of FIG. 10B. These increased ratios translate to an on-axismonochromaticity of 0.96 (M=0.96) and an off-axis monochromaticity of0.95 (M=0.95), compared to an on-axis monochromaticity of 0.94 (M=0.94)for x-ray spectrum of FIG. 14A and an off-axis monochromaticity of 0.89(M=0.89) for the x-ray spectrum of FIG. 14B, and an on-axismonochromaticity of 0.33 and an off-axis monochromaticity of 0.4 for thex-ray spectra of FIGS. 10A and 10B, respectively.

Referring again to FIGS. 15 and 16, the stars indicate the on-axis andoff-axis low energy ratio (1580 a) and high energy ratio (1580 b), aswell as the on-axis and off-axis monochromaticity (1680), respectively,of the x-ray radiation emitted using exemplary carrier 1640. As shown,the x-ray radiation exhibits essentially the same characteristicson-axis and 5 degrees off-axis. Accordingly, while exemplary carrier1740 improves both on-axis and off-axis monochromaticity, use of theexemplary carrier illustrate in FIGS. 17A-C exhibits a substantialincrease in the off-axis monochromaticity, providing substantialbenefits to x-ray imaging using monochromatic x-rays, for example, byimproving uniformity, reducing dose and enabling the use of higher x-raytube voltages to increase the mononchromatic intensity to improve thespatial resolution and ability differentiate small density variations(e.g., small tissue anomalies such as micro-calcifications in breastmaterial), as discussed in further detail below. Using carrier 1740,monochromatic x-ray radiation is emitted having a monochromaticity of atleast 0.9 across a 15 degree field of view and a monochromaticity of atleast 0.95 across a 10 degree field of view about the longitudinal axis.

It should be appreciated that the exemplary carrier described herein maybe configured to be a removable housing or may be integrated into thex-ray device. For example, one or more aspects of the exemplary carriersdescribed herein may integrated, built-in or otherwise made part anx-ray device, for example, as fixed components, as the aspects are notlimited in this respect.

As is well known, the intensity of monochromatic x-ray emission may beincreased by increasing the cathode-anode voltage (e.g., the voltagepotential between filament 1106 and primary target 1100 illustrated inFIGS. 11A and 11B) and/or by increasing the filament current which, inturn, increases the emission current of electrons emitted by thefilament, the latter technique of which provides limited control as itis highly dependent on the properties of the cathode. The relationshipbetween x-ray radiation intensity, cathode-anode voltage and emissioncurrent is shown in FIG. 20, which plots x-ray intensity, produced usinga silver (Ag) secondary target and a source-detector distance of 750 mm,against emission current at a number of different cathode-anode voltagesusing two different secondary target geometries (i.e., an Ag cone havinga 4 mm diameter base and an Ag cone having a 8 mm diameter base).

Conventionally, the cathode-anode voltage was selected to beapproximately twice that of the energy of the characteristic emissionline of the desired monochromatic x-ray radiation to be fluoresced bythe secondary target as a balance between producing sufficient highenergy broadband x-ray radiation above the absorption edge capable ofinducing x-ray fluorescence in the secondary target to produce adequatemonochromatic x-ray intensity, and producing excess high energybroadband x-ray radiation that contaminates the desired monochromaticx-ray radiation. For example, for an Ag secondary target, acathode-anode potential of 45 kV (e.g., the electron optics would be setat −45 kV) would conventionally be selected to ensure sufficient highenergy broadband x-rays are produced above the K-edge of silver (25 keV)as illustrated in FIG. 21 to produce the 22 keV Ag K monochromatic x-rayradiation shown in FIG. 19 (bottom left). Similarly, for a Sn secondarytarget, a cathode-anode potential of 50 kV would conventionally beselected to ensure sufficient high energy broadband x-rays are producedabove the K-edge of tin (29 keV) as illustrated in FIG. 21 to producethe 25 keV Sn K monochromatic x-ray radiation shown in FIG. 19 (bottomright). This factor of two limit on the cathode-anode voltage wasconventionally followed to limit the high energy contamination of themonochromatic x-rays emitted from the x-ray apparatus.

The inventor has recognized that the techniques described herein permitthe factor of two limit to be eliminated, allowing high cathode-anodevoltages to be used to increase mononchromatic x-ray intensity withoutsignificantly increasing broadband x-ray radiation contamination (i.e.,without substantial decreases in monochromaticity). In particular,techniques for blocking broadband x-ray radiation, including theexemplary secondary target carriers developed by the inventors can beused to produce high intensity monochromatic radiation while maintainingexcellent monochromaticity. For example, FIG. 22 illustrates the on-axismonochromaticity 2200 a and the off-axis monochromaticity 2200 b for anumber of cathode-anode voltages (primary voltage) with a Sn secondarytarget using exemplary carrier 1740 developed by the inventor.Similarly, FIG. 23 illustrates the on-axis monochromaticity 2300 a andthe off-axis monochromaticity 2300 b for a number of cathode-anodevoltages (primary voltage) with an Ag secondary target using exemplarycarrier 1740 developed by the inventor. As shown, a high degree ofmonochromaticity is maintained across the illustrated range of highvoltages, varying by only 1.5% over the range illustrated. Thus, highervoltages can be used to increase the monochromatic x-ray intensity(e.g., along the lines shown in FIG. 20) without substantially impactingmonochromaticity. For example, monochromatic x-ray radiation of over 90%purity (M>0.9) can be generated using a primary voltage up to andexceeding 100 KeV, significantly increasing the monochromatic x-rayintensity.

According to some embodiments, a primary voltage (e.g., a cathode-anodevoltage potential, such as the voltage potential between filament 1106and primary target 1110 of x-ray tube 1150 illustrated in FIGS. 11A and11B) greater than two times the energy of the desired monochromaticx-ray radiation fluoresced from a given target is used to generatemonochromatic x-ray radiation. According to some embodiments, a primaryvoltage greater than or equal to approximately two times and less thanor equal to approximately three times the energy of the desiredmonochromatic x-ray radiation fluoresced from a given target is used togenerate monochromatic x-ray radiation. According to some embodiments, aprimary voltage greater than or equal to approximately three times andless than or equal to approximately four times the energy of the desiredmonochromatic x-ray radiation fluoresced from a given target is used togenerate monochromatic x-ray radiation. According to some embodiments, aprimary voltage greater than or equal to approximately four times andless than or equal to approximately five times the energy of the desiredmonochromatic x-ray radiation fluoresced from a given target is used togenerate monochromatic x-ray radiation. According to some embodiments, aprimary voltage greater than or equal to five times greater the energyof the desired monochromatic x-ray radiation fluoresced from a giventarget is used to generate monochromatic x-ray radiation. In each case,x-ray radiation having monochromaticity of greater than or equal to 0.9,on and off axis across the field of view may be achieved, though itshould be appreciated that achieving those levels of monochromaticity isnot a requirement.

The inventor has recognized the geometry of the x-ray tube maycontribute to broadband x-ray radiation contamination. The inventor hasappreciated that the electron optics of an x-ray tube may be improved tofurther reduce the amount of broadband x-ray radiation that is generatedthat could potentially contaminate the monochromatic x-rays emitted froman x-ray device. Referring again to FIGS. 11A and 11B, x-ray device 1100includes electron optics 1105 configured to generate electrons thatimpinge on primary target 1110 to produce broadband x-ray radiation. Theinventor has developed electron optics geometry configured to reduceand/or eliminate bombardment of surfaces other than the primary targetwithin the vacuum enclosure. This geometry also reduces and/oreliminates parasitic heating of other surfaces that would have to beremoved via additional cooling in conventional systems.

As an example, the geometry of electron optics 1105 is configured toreduce and/or eliminate bombardment of window portion 1130 and/or othersurfaces within vacuum tube 1150 to prevent unwanted broadband x-rayradiation from being generated and potentially emitted from the x-raytube to degrade the monochromaticity of the emitted x-ray radiationspectrum. In the embodiment illustrated in FIGS. 11A and 11B, electronoptics 1105 comprises a filament 1106, which may be generally toroidalin shape, and guides 1107, 1108 and/or 1109 positioned on the inside andoutside of the toroidal filament 1106. For example, guides 1107, 1108,1109 may be positioned concentrically with the toroidal filament 1106(e.g., an inner guide 1107 positioned within the filament torus and anouter guides 1108 and 1109 positioned around the filament torus) toprovide walls on either side of filament 1106 to prevent at least someelectrons from impinging on surfaces other than primary target 1110, asdiscussed in further detail below.

According to some embodiments, electronic optics 105 is configured tooperate at a high negative voltage (e.g., 40 kV, 50 kV, 60 kV, 70 kV, 80kV, 90 kV or more). That is, filament 1106, inner guide 1107 and outerguides 1108, 1109 may all be provided at a high negative potentialduring operation of the device. As such, in these embodiments, primarytarget 1110 may be provided at a ground potential so that electronsemitted from filament 1106 are accelerated toward primary target 1110.However, the other components and surfaces of x-ray tube within thevacuum enclosure are typically also at ground potential. As a result,electrons will also accelerate toward and strike other surfaces of x-raytube 1150, for example, the transmissive interface between the insideand outside of the vacuum enclosure (e.g., window 1130 in FIGS. 11a and11b ). Using conventional electron optics, this bombardment ofunintended surfaces produces broadband x-ray radiation that contributesto the unwanted broadband spectrum emitted from the x-ray device andcauses undesirable heating of the x-ray tube. The inventor appreciatedthat this undesirable bombardment of surfaces other than primary target1110 may be reduced and/or eliminated using inner guide 1107 and outerguides 1108 and/or 1109 that provide a more restricted path forelectrons emitted by filament 1106.

According to some embodiments, guides 1107-1109 are cylindrical in shapeand are arranged concentrically to provide a restricted path forelectrons emitted by filament 1106 that guides the electrons towardsprimary target 1110 to prevent at least some unwanted bombardment ofother surfaces within the vacuum enclosure (e.g., reducing and/oreliminating electron bombardment of window portion 1130). However, itshould be appreciated that the guides used in any given implementationmay be of any suitable shape, as the aspects are not limited in thisrespect. According to some embodiments, guides 1107, 1108 and/or 1109comprise copper, however, any suitable material that is electricallyconducting (and preferably non-magnetic) may be used such as stainlesssteel, titanium, etc. It should be appreciated that any number of guidesmay be used. For example, an inner guide may be used in conjunction witha single outer guide (e.g., either guide 1108 or 1109) to provide a pairguides, one on the inner side of the cathode and one on the outer sideof the cathode. As another example, a single inner guide may be providedto prevent at least some unwanted electrons from bombarding theinterface between the inside and outside of the vacuum tube (e.g.,window portion 1130 in FIGS. 11A and 11B), or a single outer guide maybe provide to prevent at least some unwanted electrons from bombardingother internal surface of the vacuum tube provides. Additionally, morethan three guides may be used to restrict the path of electrons to theprimary target to reduce and/or eliminate unwanted bombardment ofsurfaces within the vacuum enclosure, as the aspects are not limited inthis respect.

FIGS. 24A and 24B illustrate a cross-section of a monochromatic x-raysource 2400 with improved electron optics, in accordance with someembodiments. In the embodiment illustrated, there is a 80 kV is thepotential between the cathode and the anode. Specifically, a tungstentoroidal cathode 2406 is bias at −80 kV and a gold-coated tungstenprimary target 2410 is at a ground potential. A copper inner guide 2407and an outer copper guides 2408 and 2409 are also provided at −80 kV toguide electrons emitted from the cathode to prevent at least someelectrons from striking surfaces other than primary target 2410 toreduce the amount of spurious broadband x-ray radiation. Monochromaticx-ray source 2400 uses a silver secondary target 2420 and a berylliuminterface component 2430. FIG. 24B illustrates the electron trajectoriesbetween the toroidal cathode and the primary target when themonochromatic x-ray source 2400 is operated. FIGS. 25 and 26 illustratethe locus of points where the electrons strike primary target 2410,demonstrating that the guides prevent electrons from striking theinterface component 2430 in this configuration. FIG. 27 illustrates amonochromatic x-ray source including a hybrid interface component havingtransmissive portion of beryllium and a blocking portion of tungstenthat produces monochromatic x-ray radiation of 97% purity (M=0.97) whencombined with other techniques described herein (e.g., using theexemplary carriers described herein). FIG. 28 illustrates an alternativeconfiguration in which the cathode is moved further away from theprimary target, resulting in divergent electron trajectories and reducedmonochromaticity.

The monochromatic x-ray sources described herein are capable ofproviding relatively high intensity monochromatic x-ray radiation havinga high degree of monochromaticity, allowing for relatively shortexposure times that reduce the radiation dose delivered to a patientundergoing imaging while obtaining images with high signal-to-noiseratio. Provided below are results obtained using techniques describedherein in the context of mammography. These results are provided toillustrate the significant improvements that are obtainable using one ormore techniques described herein, however, the results are provided asexamples as the aspects are not limited for use in mammography, nor arethe results obtained requirements on any of the embodiments describedherein.

FIG. 29 illustrates a mammographic phantom (CIRS Model 011a) 2900 usedto test aspects of the performance of the monochromatic x-ray devicedeveloped by the inventor incorporating techniques described herein.Phantom 2900 includes a number of individual features of varying sizeand having different absorption properties, as illustrated by theinternal view of phantom 2900 illustrated in FIG. 29. FIG. 30 highlightssome of the embedded features of phantom 2900, including the lineararray of 5 blocks, each 1 cm thick and each having a compositionsimulating different densities of breast tissue. The left most blocksimulates 100% glandular breast tissue, the right most, 100% adipose(fat) tissue and the other three have a mix of glandular and adiposewith ratios ranging from 70:30 (glandular:adipose) to 50:50 to 30:70.All 5 blocks are embedded in the phantom made from a 50:50 glandular toadipose mix. The total thickness of the phantom is 4.5 cm.

FIG. 30 also shows a schematic description of the imaging process in onedimension as the x-ray beam enters the phantom, passes through theblocks and the phantom on their way to the imaging detector where thetransmitted x-ray intensity, is converted into an integrated value ofGray counts. (The intensity in this case is the sum of the x-rayenergies reaching each detector pixel. The electronics in each pixelconvert this energy sum into a number between 0 and 7000, where 7000represents the maximum energy sum allowable before the electronicssaturate. The number resulting from this digital conversion is termed aGray count).

The data shown by the red horizontal line in a) of FIG. 30 is the x-rayintensity, B, measured through the background 50:50 glandular-adiposemixture. The data shown by the black curve is the x-ray intensity, W,transmitted through the 50:50 mix and the 1 cm blocks. The varying stepsizes represent different amounts of x-ray absorption in the blocks dueto their different compositions. Plot b) in FIG. 30 defines the signal,S, as W-B and plot c) of FIG. 30 defines the contrast as S/B. The figureof merit that is best used to determine the detectability of an imagingsystem is the Signal-to-Noise Ratio, SNR. For the discussion here, theSNR is defined as S/noise, where the noise is the standard deviation ofthe fluctuations in the background intensity shown in plot a) of FIG.30. Images produced using techniques described herein and may with 22keV x-rays and 25 keV x-rays and presented herein and compared to theSNR values with those from a commercial broad band x-ray mammographymachine.

Radiation exposure in mammographic examinations is highly regulated bythe Mammography Quality Standards Act (MQSA) enacted in 1994 by the U.S.Congress. The MQSA sets a limit of 3 milliGray (mGy) for the meanglandular dose (mgd) in a screening mammogram; a Gray is ajoule/kilogram. This 3 mGy limit has important ramifications for theoperation of commercial mammography machines, as discussed in furtherdetail below. Breast tissue is composed of glandular and adipose (fatty)tissue. The density of glandular tissue (ρ=1.03 gm/cm³) is not verydifferent from the density of adipose tissue (ρ=0.93 gm/cm³) which meansthat choosing the best monochromatic x-ray energy to optimize the SNRdoes not depend significantly on the type of breast tissue. Instead, thechoice of monochromatic energy for optimal imaging depends primarily onbreast thickness. A thin breast will attenuate fewer x-rays than a thickbreast, thereby allowing a more significant fraction of the x-rays toreach the detector. This leads to a higher quality image and a higherSNR value. These considerations provide the major rationale forrequiring breast compression during mammography examinations with aconventional, commercial mammography machine.

Imaging experiments were conducted the industry-standard phantomillustrated in FIG. 29, which has a thickness of 4.5 cm and isrepresentative of a typical breast under compression. Phantom 2900 has auniform distribution of glandular-to-adipose tissue mixture of 50:50.The SNR and mean glandular dose are discussed in detail below for ORSphantom images obtained with monochromatic energies of 22 keV and 25keV. Experiments were also conducted with a double phantom, asillustrated in FIG. 32, to simulate a thick breast under compressionwith a thickness of 9 cm. The double phantom also has a uniformdistribution of glandular-to-adipose tissue mixture of 50:50. The SNRand mean glandular dose are presented for the double phantom using amonochromatic energy of 25 keV. The high SNR obtained on this model of athick breast demonstrates that monochromatic x-rays can be used toexamine women with reduced compression or no compression at all, since,typically, a compressed breast of 4.5 cm thickness is equivalent to anuncompressed breast of 8-9 cm thickness, as discussed in further detailbelow.

The experiments demonstrate that the mean glandular dose for themonochromatic measurements is always lower than that of the commercialmachine for the same SNR. Stated in another way, the SNR for themonochromatic measurements is significantly higher than that of thecommercial machines for the same mean glandular dose. Thus,monochromatic X-ray mammography provides a major advance overconventional broadband X-ray mammographic methods and has significantimplications for diagnosing breast lesions in all women, and especiallyin those with thick or dense breast tissue. Dense breasts arecharacterized by non-uniform distributions of glandular tissue; thisnon-uniformity or variability introduces artifacts in the image andmakes it more difficult to discern lesions. The increased SNR thatmonochromatic imaging provides makes it easier to see lesions in thepresence of the inherent tissue variability in dense breasts, asdiscussed in further detail below.

FIG. 31 illustrates images of phantom 2900 obtained from a monochromaticx-ray source described herein using monochromatic Ag K (22 keV) and Sn K(25 keV) x-rays and an image from a conventional commercial mammographymachine that uses broad band emission, along with respective histogramsthrough the soft tissue blocks. The image from the commercial machine isshown in (a) of FIG. 31. The SNR for the 100% glandular block is 8.4 andthe mean glandular dose (mgd) is 1.25 mGy (1 Gy=1 joule/kgm). Image (b)in FIG. 31 illustrates a monochromatic image using 22 keV x-rays andimage (c) in FIG. 31 was obtained with 25 keV X-rays. The mean glandulardoses for the 100% glandular block measured with 22 keV is 0.2 mGy andthat measured with 25 keV is 0.08 mGy, and the SNR values are 8.7 forboth energies. To achieve the same SNR as the commercial machine, themonochromatic system using 22 keV delivers a dose that is 6.7 timeslower and using 25 keV delivers a dose that is 15 times lower.

The dose reduction provided by the monochromatic X-ray technology offerssignificantly better diagnostic detectability than the conventionalbroad band system because the SNR can be increased by factors of 3 to 6times while remaining well below the regulatatory dose limit of 3 mGyfor screening. For example, the SNR value for the 22 keV images would be21.8 at the same dose delivered by the commercial machine (1.25 mGy) and32 for a dose of 2.75 mGy. Similarly, using the 25 keV energy, the SNRvalues would be 34 and 51 for mean glandular doses of 1.25 mGy and 2.75mGy, respectively. This significantly enhanced range in SNR has enormousadvantages for diagnosing women with dense breast tissue. As mentionedearlier, such tissue is very non-uniform and, unlike the uniformproperties of the phantoms and women with normal density tissue, thevariability in glandular distribution in dense breast introducesartifacts and image noise, thereby making it more difficult to discernlesions. The higher SNR provided by techniques describe herein canovercome these problems.

The monochromatic x-ray device incorporating the techniques describedherein used to produce the images displayed here is comparable in sizeand footprint of a commercial broadband x-ray mammography system,producing for the first time low dose, high SNR, uniform images of amammographic phantom using monochromatic x-rays with a degree ofmonochromaticity of 95%. In fact, conventional monochromatic x-rayapparatus do not even approach these levels of monochromaticity.

To simulate thick breast mammography, a model for thick breast tissuewas created by placing two phantoms on top of each other (totalthickness 9.0 cm), the 18-220 ACR Mammography Accreditation Phantom(3200) placed on top of the CIRS Model 011A phantom (2900), as shown inFIG. 32. For this series of experiments, 25 keV x-rays were selected tooptimize the transmission while maintaining good contrast in the softtissue represented by the 1 cm array of blocks embedded on the CIRSphantom. The images for the 25 keV monochromatic x-rays are compared tothe images obtained from the same commercial broad band mammographymachine used in the previous experiment. The resulting images aredisplayed in FIG. 33, along with the histograms of the contrast throughthe soft tissue blocks.

The image quality for the thick breast tissue is superior to anythingobtainable with current commercial broad band systems. The dosedelivered by the commercial machine is 2.75 mGy and only achieves a SNRof 3.8 in the 100% glandular block. The monochromatic image in FIG. 33has a SNR=7.5 for a dose of 0.43 mGy. The dose required for thecommercial broad band X-ray system to reach a SNR of 8.5, the acceptedvalue of radiologists for successful detection in thinner 4.5 cm thicktissue would be 14 mGy, 11 times higher than the commercial dose used toimage normal density breast tissue (1.25 mGy). This is prohibitivelyhigh and unsafe for screening and 4.7 times higher than the regulatedMQSA screening limit. On the other hand, the required dose from themonochromatic system to achieve a SNR=8.5 is only 0.54 mGy, 26 timeslower than that required by the commercial machine. The dose requiredusing monochromatic x-rays is safe, more than 5 times lower than theregulatory limit, and still 2.5 times lower than the dose for normalthickness, 4.5 cm breasts using the commercial broad band x-raymammography machine. Comparing the monochromatic X-ray and thecommercial broad band X-ray machines at close to the maximum allowedexposure (2.75 mGy), the monochromatic technology provides 5 timeshigher SNR. The above discussion is summarized schematically in FIG. 34.

The measurements on the 9 cm thick breast phantom show that themonochromatic techniques described herein facilitate elimination ofbreast compression during mammography screening. A 4.5 cm compressedbreast could be as thick at 9 cm when uncompressed. Whereas thecommercial machine loses sensitivity as the breast thickness increasesbecause it cannot increase the dose high enough to maintain the SNR andstill remain below the regulated dose limit, the monochromatic x-raysystem very easily provides the necessary SNR. As an example, of amonochromatic mammography procedure, a woman may lie prone on a clinictable designed to allow her breasts to extend through cutouts in thetable. The monochromatic x-ray system may be designed to direct thex-rays parallel to the underside of the table. The table alsofacilitates improved radiation shielding for the patient byincorporating a layer of lead on the underside of the table's horizontalsurface.

The inventor has recognized that the spatial resolution of the geometryof the monochromatic x-ray device described herein is excellent formammographic applications. According to some embodiments, themonochromatic x-ray system has a source-to-detector distance of 760 mm,a secondary target cone with a 4 mm base diameter and 8 mm height, andan imaging detector of amorphous silicon with pixel sizes of 85 microns.This exemplary monochromatic x-ray device using the techniques describedherein can easily resolve microcalicifications with diameters of 100-200microns in the CIRS and ACR phantoms. FIGS. 35 and 36 illustrate imagesand associated histograms obtained using this exemplary monochromaticx-ray radiation device compared to images obtained using the samecommercial device. The microcalcifications measured in the doubleACR-CIRS phantom (stacked 2900 and 3200 phantoms) experiments describedearlier using the monochromatic 25 keV x-ray lines have a SNR that is50% higher than the SNR for the commercial machine and its meanglandular dose (mgd) is 6 times lower for these images. If one were tomake the monochromatic SNR the same as that measured in the commercialmachine, then the monochromatic mean glandular does (mgd) would beanother factor of 2 times smaller for a total of 11 times lower.

Simple geometric considerations indicate that the effective projectedspot size of the secondary cone is 1-2 mm. FIG. 37 illustrateshistograms of the measured intensity scans through line-pair targetsthat are embedded in the CIRS phantom. The spacing of the line-partargets ranges from 5 lines per mm up to 20 lines per mm. The top fourhistograms show that the scans for 18 keV, 21 keV, 22 keV and 25 keVenergies using a 4 mm secondary cone described briefly above can discernalternating intensity structure up to 9 lines per mm which is consistentwith a spatial resolution FWHM of 110 microns. The 18 keV energy canstill discern structure at 10 lines per mm. The bottom histogram in FIG.37 is an intensity scan through the same line-pair ensemble using acommonly used commercial broad band mammography system. The commercialsystem's ability to discern structure fails beyond 8 lines per mm. Thisperformance is consistent with the system's modulation transfer function(MTF), a property commonly used to describe the spatial frequencyresponse of an imaging system or a component. It is defined as thecontrast at a given spatial frequency relative to low frequencies and isshown in FIG. 38. The value of 0.25 at 9 lines/mm is comparable to othersystems with direct detector systems and better than flat paneldetectors.

According to some embodiments, the exemplary monochromatic systemdescribed herein was operated with up to 2000 watts in a continuousmode, i.e., the primary anode is water-cooled, the high voltage andfilament current are on continuously and images are obtained using atimer-controlled, mechanical shutter. The x-ray flux data in FIG. 20together with the phantom images shown in FIGS. 31 and 33 providescaling guidelines for the power required to obtain a desired signal tonoise for a specific exposure time in breast tissue of differentcompression thicknesses. Using a secondary material of Ag, 4 mm and 8 mmcone assemblies are compared for a compressed thickness of 4.5 cm and50:50 glandular-adipose mix) in FIG. 39. The power requirements for acompressed thickness of 9 cm (50:50 glandular-adipose mix) as defined byexperiments described above are compared in FIG. 40 for the 4 mm, 8 mmcones made from Sn.

The results indicate that a SNR of 8.5 obtained in a measurement of the100% glandular block embedded in the ORS phantom of normal breastdensity compressed to 4.5 cm can be achieved in a 5 second exposureexpending 9.5 kW of power in the primary using the 4 mm cone (FIG. 39top); 3.7 kW are needed if one uses the 8 mm cone (FIG. 39 bottom). Inboth of these cases, the source-to-detector (S-D) is 760 mm. If 2 secare required, 9.2 kW are needed if an 8 mm cone is used or a 4 mm conecan be used at a source-to-detector (S-D) distance of 471 mm instead of760 mm. Since the spatial resolution dependence is linear with S-D, thenmoving the 4 mm cone closer to the sample will only degrade the spatialresolution by 1.6 times, but it will still be better than the 8 mm coneat 760 mm. In general, there is a trade-off between spatial resolutionand exposure time that will determine whether the 4 mm or 8 mmembodiments at the two source-to-detector distances best suit anapplication. This data serves as guides for designing monochromaticx-ray sources and do not exclude the possibilities for a variety ofother target sizes and source-to-detector distances.

For thick breast tissue compressed to 9 cm, the dependency of the SNR onpower is shown in FIG. 40. A 7 sec exposure can produce a SNR of 8.5 at11 kW using a 4 mm Sn cone at a source-to-detector distance of 471 mm orwith a 8 mm cone at 760 mm. Conventional broad band commercialmammography systems would have to deliver a 14 mGy dose to achieve thissame SNR whereas the monochromatic system at 25 keV would only deliver0.54 mGy, a factor of 26 times lower and still 2.3 times lower than theconventional dose of 1.25 mGy delivered by a commercial machine inscreening women with normal density breast tissue compressed to 4.5 cm.

The inventor has recognized that maximizing the monochromatic X-rayintensity in a compact X-ray generator may be important for applicationsin medical imaging. Increased intensity allows shorter exposures whichreduce motion artifacts and increase patient comfort. Alternatively,increased intensity can be used to provide increased SNR to enable thedetection of less obvious features. There are three basic ways toincrease the monochromatic flux: 1) maximizing fluorescence efficiencythrough the geometry of the target, 2) enhance the total power input onthe primary in a steady state mode and 3) increase the total power inputon the primary in a pulsed mode.

To increase the power and further decrease the exposure times, powerlevels of 10 kW-50 kW may be used. For example, an electron beam in highpower commercial medical x-ray tubes (i.e., broadband x-ray tubes) hasapproximately a 1×7 mm fan shape as it strikes an anode that is rotatingat 10,000 rpm. Since the anode is at a steep angle to the electron beam,the projected spot size in the long direction as seen by the viewer isreduced to about 1 mm. For an exposure of 1 sec, once can consider theentire annulus swept out by the fan beam as the incident surface forelectron bombardment. For a 70 mm diameter anode, this track length is210 mm, so the total incident anode surface area is about 1400 mm². Forthe monochromatic system using a conical anode with a 36 mm diameter anda truncated height of 6 mm, the total area of incidence for theelectrons is 1000 mm². Therefore, it should be straightforward to make a1 sec exposure at a power level that is 70% of the power of strongmedical sources without damaging the anode material; 100 kW is a typicalpower of the highest power medical sources. Assuming a very conservativevalue that is 50% of the highest power, an anode made of a compositematerial operating at 50 kW should be achievable for short exposures.This is more power than is needed for thick and/or dense breastdiagnostics but offers significant flexibility if reducing the effectivesize of the secondary cone becomes a priority.

A one second exposure at 50 kW generates 50 kJ of heat on the anode. Ifthe anode is tungsten, the specific heat is 0.134 J/g/K. To keep thetemperature below 1000° C. in order not to deform or melt the anode, theanode mass needs to be at least 370 gm. An anode of copper coated with athick layer of gold would only have to be 130 gm. These parameters canbe increased by at least 2-3 times without seriously changing the sizeor footprint of the source. For repeat exposures or for longerexposures, the anode in this system can be actively cooled whereas therotating anode system has to rely on anode mass for heat storage andinefficient cooling through a slip-ring and slow radiative transfer ofheat out of the vacuum vessel. The monochromatic x-ray systems describedabove can be actively cooled with water.

According to some embodiments, the primary anode material can be chosento maximize the fluorescent intensity from the secondary. In the teststo date, the material of the primary has been either tungsten (W) orgold (Au). They emit characteristic K emission lines at 59 keV and 68keV, respectively. These energies are relatively high compared to theabsorption edges of silver (Ag; 25.6 keV) or tin (Sn; 29 keV) therebymaking them somewhat less effective in inducing x-ray fluorescence inthe Ag or Sn secondary targets. These lines may not even be excited ifthe primary voltage is lower than 59 keV. In this situation only theBremsstrahlung induces the fluorescence. Primary material can be chosenwith characteristic lines that are much closer in energy to theabsorption edges of the secondary, thereby increasing the probability ofx-ray fluorescence. For example, elements of barium, lanthanum, cerium,samarium or compounds containing these elements may be used as long asthey can be formed into the appropriate shape. All have melting pointsabove 1000° C. If one desires to enhance production of monochromaticlines above 50 keV in the most efficient way, higher Z elements areneeded. For example, depleted uranium may be used (K line=98 keV) toeffectively induce x-ray fluorescence in Au (absorption edge=80.7 keV).Operating the primary at 160 kV, the Bremsstrahlung plus characteristicuranium K lines could produce monochromatic Au lines for thorasic/chestimaging, cranial imaging or non-destructive industrial materialsanalysis.

For many x-ray imaging applications including mammography, the x-raydetector is an imaging array that integrates the energies of theabsorbed photons. All spectroscopic information is lost. If aspectroscopic imager is available for a particular situation, thesecondary target could be a composite of multiple materials.Simultaneous spectroscopic imaging could be performed at a minimum oftwo energies to determine material properties of the sample. Even if animaging detector with spectral capability were available for use with abroad-band source used in a conventional x-ray mammography system forthe purpose of determining the chemical composition of a suspiciouslesion, the use of the spectroscopic imager would not reduce the dose tothe tissue (or generically the sample) because the broad band sourcedelivers a higher dose to the sample than the monochromatic spectrum.

Contrast-enhanced mammography using monochromatic x-ray radiation issuperior to using the broad band x-ray emission. It can significantlyincrease the image detail by selectively absorbing the monochromaticX-rays at lower doses. The selective X-ray absorption of a targetedcontrast agent would also facilitate highly targeted therapeutic X-raytreatment of breast tumors. In the contrast enhanced digitalmammographic imaging conducted to date with broad band x-ray emissionfrom conventional x-ray tubes, users try to take advantage of theincreased absorption in the agent, such as iodine, by adjusting thefiltering and increasing the electron accelerating voltage to producesufficient x-ray fluence above the 33 keV K absorption edge of iodine.FIG. 41 shows the mass absorption curves for iodine as a function ofx-ray energy. The discontinuous jumps are the L and K absorption edges.The contrast media will offer greater absorption properties if the broadband spectra from conventional sources span an energy range thatincorporates these edges. As a result, detectability should improve.

Monochromatic radiation used in the mammographic system discussed hereoffers many more options for contrast-enhanced imaging. Ordinarily, onecan select a fluorescent target to produce a monochromatic energy thatjust exceeds the iodine absorption edge. In this sense, themonochromatic x-ray emission from the tube is tuned to the absorptioncharacteristics of the contrast agent. To further improve thesensitivity, two separate fluorescent secondary targets may be chosenthat will emit monochromatic X-rays with energies that are below andabove the absorption edge of the contrast agent. The difference inabsorption obtained above and below the edge can further improve theimage contrast by effectively removing effects from neighboring tissuewhere the contrast agent did not accumulate. Note that the majority ofx-ray imaging detectors currently used in mammography do not have theenergy resolution to discriminate between these two energies if theyirradiate the detector simultaneously; these two measurements must bedone separately with two different fluorescent targets in succession.This is surely a possibility and is incorporated in our system.

Since the contrast agent enhances the x-ray absorption relative to thesurrounding tissue, it is not necessary to select a monochromatic energyabove the K edge to maximize absorption. For example, FIG. 41 shows thatthe absorption coefficient for the Pd Kα 21.175 keV energy, which isbelow the K edge, is comparable to the absorption coefficient of the NdKα 37.36 keV energy which is above the K edge. As long as the atoms ofthe contrast agent are sufficiently heavier (atomic number, Z>45) thanthe those in the surrounding tissue (C, O, N, P, S; Z<10 and traceamounts of Fe, Ni, Zn, etc., Z<30), the monochromatic x-ray techniqueincreases the potential choices for contrast agents in the future. Thesecondary targets of Pd, Ag and Sn are perfect options for thisapplication. Using monochromatic energies below the absorption edge ofiodine, for example, takes better advantage of the quantum absorptionefficiency of a typical mammographic imaging detector. The absorption at37 keV (above the iodine edge) is about 2 times lower than at 22 keV(below the edge). The lower energy may also prove to have betterdetectability in the surrounding tissue simultaneously. FIG. 42 shows alinear set of 3 drops of Oxilan 350, an approved iodine contrast agentmanufactured by Guerbet superimposed on the the ACR phantom. The amountof iodine in each of the drops ˜1 mg iodine.

Having thus described several aspects and embodiments of the technologyset forth in the disclosure, it is to be appreciated that variousalterations, modifications, and improvements will readily occur to thoseskilled in the art. Such alterations, modifications, and improvementsare intended to be within the spirit and scope of the technologydescribed herein. For example, those of ordinary skill in the art willreadily envision a variety of other means and/or structures forperforming the function and/or obtaining the results and/or one or moreof the advantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the embodimentsdescribed herein. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific embodiments described herein. It is, therefore, to beunderstood that the foregoing embodiments are presented by way ofexample only and that, within the scope of the appended claims andequivalents thereto, inventive embodiments may be practiced otherwisethan as specifically described. In addition, any combination of two ormore features, systems, articles, materials, kits, and/or methodsdescribed herein, if such features, systems, articles, materials, kits,and/or methods are not mutually inconsistent, is included within thescope of the present disclosure.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively.

What is claimed is:
 1. A monochromatic x-ray device comprising: anelectron source configured to emit electrons; a primary targetconfigured to produce broadband x-ray radiation in response to incidentelectrons from the electron source; a secondary target configured togenerate monochromatic x-ray radiation via fluorescence in response toincident broadband x-ray radiation; and a housing for the secondarytarget comprising an aperture through which monochromatic x-rayradiation from the secondary target is emitted, the housing configuredto position the secondary target so that at least some of the broadbandx-ray radiation emitted by the primary target is incident on thesecondary target so that, when the monochromatic x-ray device isoperated, monochromatic x-ray radiation is emitted via the aperturehaving a monochromaticity of greater than or equal to 0.7 across a fieldof view of at least approximately 15 degrees.
 2. The monochromatic x-raydevice of claim 1, wherein, when the device is operated, monochromaticx-ray radiation is emitted via the aperture having a monochromaticity ofgreater than or equal to 0.8 across a field of view of at leastapproximately 15 degrees.
 3. The monochromatic x-ray device of claim 1,wherein, when the device is operated, monochromatic x-ray radiation isemitted via the aperture having a monochromaticity of greater than orequal to 0.9 across a field of view of at least approximately 15degrees.
 4. The monochromatic x-ray device of claim 1, wherein, when thedevice is operated, monochromatic x-ray radiation is emitted via theaperture having a monochromaticity of greater than or equal to 0.95across a field of view of at least approximately 15 degrees.
 5. Themonochromatic x-ray device of claim 1, wherein the housing comprises: atransmissive portion configured to allow broadband x-ray radiation to betransmitted to the secondary target when present; and a blocking portionconfigured to absorb broadband x-ray radiation.
 6. The monochromaticx-ray device of claim 5, wherein the transmissive portion comprises atleast a first transmissive material substantially transparent to thebroadband x-ray radiation emitted by the broadband x-ray source.
 7. Themonochromatic x-ray device of claim 6, wherein the transmissive portioncomprises at least one of aluminum, carbon, carbon fiber, boron, boronnitride, beryllium oxide, silicon and silicon nitride.
 8. Themonochromatic x-ray device of claim 5, wherein the blocking portioncomprising at least a first blocking material substantially opaque tothe broadband x-ray radiation emitted by the broadband x-ray source. 9.The monochromatic x-ray device of claim 8, wherein the blocking portioncomprises at least one of lead, tungsten, tantalum, rhenium, platinumand gold.
 10. The monochromatic x-ray device of claim 5, wherein thehousing is configured such that, when the secondary target isaccommodated within the housing, at least a first portion of thesecondary target is positioned within the transmissive portion of thehousing and a second portion of the secondary target is positionedwithin the blocking portion of the housing.
 11. The monochromatic x-raydevice of claim 10, wherein the transmissive portion comprises a firstcylindrical portion configured to accommodate the secondary target andthe blocking portion comprises a second cylindrical portion configuredto overlap the second portion of the secondary target when the firstcylindrical portion accommodates the secondary target.
 12. Themonochromatic x-ray device of claim 11, wherein the secondary target isconical, wherein the first cylindrical portion is configured toaccommodate at least a portion of the base of the conical secondarytarget, and wherein the second cylindrical portion is configured tooverlap at least a portion of a tip of the conical secondary target. 13.The monochromatic x-ray device of claim 11, wherein the secondcylindrical portion is configured to overlap at least 1 mm of thesecondary target when the first cylindrical portion accommodates thesecondary target.
 14. The monochromatic x-ray device of claim 11,wherein the second cylindrical portion is configured to overlap at least2 mm of the secondary target when the first cylindrical portionaccommodates the secondary target.
 15. The monochromatic x-ray device ofclaim 11, wherein the second cylindrical portion is configured tooverlap at least 3 mm of the secondary target when the first cylindricalportion accommodates the secondary target.
 16. The monochromatic x-raydevice of claim 4, wherein the housing is configured such that, when thesecondary target is accommodated within the housing, the secondarytarget is positioned exclusively within the transmissive portion. 17.The monochromatic x-ray device of claim 4, wherein the transmissiveportion comprises a first cylindrical portion configured to accomodateat least a portion of the secondary target and the blocking portioncomprises: a second cylindrical portion having a first diameter, thesecond cylindrical portion positioned adjacent the first cylindricalportion; and an annular portion having a second diameter larger than thefirst diameter, the annular portion positioned adjacent the secondcylindrical portion and configured to emit monochromatic x-ray radiationvia an aperture provide through the annular portion.
 18. Themonochromatic x-ray device of claim 17, wherein the first cylindricalportion is positioned on a proximal end of the housing that isconfigured to be inserted into a receptacle of the broadband x-raysource, and the annular portion is positioned on a distal end of thecarrier from which monochromatic x-ray radiation is emitted.
 19. Themonochromatic x-ray device of claim 1, wherein the housing is configuredto be removable.